ments at Chatham and Portsmouth. These results are conformable with his original theoretical views.

"Sheets of copper, defended by from to Tobo part of their surface of zinc, malleable and cast iron, were exposed, for many weeks, in the flow of the tide in Portsmouth harbour, and their weights ascertained before and after the experiment. When the metallic protector was from to there was no corrosion nor decay of the copper; with smaller quantities, such as from to the copper underwent a loss of weight, which was greater in proportion as the protector was smaller; and as a proof of the universality of the principle, it was found that even Too part of cast iron saved a certain proportion of the copper.

"The sheeting of boats and ships, protected by the contact of zinc, cast and malleable iron in different proportions, compared with those of similar boats and sides of ships unprotected, exhibited bright surfaces, whilst the unprotected copper underwent rapid corrosion, becoming first red, then green, and losing a part of its substance in scales.

"Fortunately, in the course of these experiments, it has been proved that cast iron, the substance which is cheapest and most easily procured, is likewise most fitted for the protection of the copper. It lasts longer than malleable iron, or zinc; and the plumbaginous substance, which is left by the action of sea water upon it, retains the original form of the iron, and does not impede the electrical action of the remaining metal.

"I had anticipated the deposition of alkaline substances in certain cases upon the negatively electrical copper. This has actually happened. Some sheets of copper, that have been exposed nearly four months to the action of sea water, defended by from to of their surface of zinc and iron, have become coated with a white matter, which, on analysis, has proved to be principally carbonated lime, and carbonate and hydrate of magnesia. The same thing has occurred with two harbour boats, one of which was defended by a band of zinc, the other by a band of iron, equal to about of the surface of the copper.

"These sheets and boats remained perfectly clean for many weeks, as long as the metallic surface of the copper was exposed; but lately, since it has become coated with carbonate of lime and magnesia, weeds have adhered to these coatings, and insects collected on them; but on the sheets of copper, defended by quantities of cast iron and zinc, bearing a proportion below the electrical power of the copper being

less negative, more neutralised, and nearly in equilibrio with that of the menstruum, no such effect of deposition of alkaline matter or adherence of weeds has taken place, and the surface, though it has undergone a slight degree of solution, has remained perfectly clean: a circumstance of great importance, as it points out the limits of protection; and makes the application of a very small quantity of the oxidable metal more advantageous in fact than that of a larger one.

"The wear of cast iron is not so rapid; but that a mass of two or three inches in thickness will last for some years. At least the consumption in experiments which have been going on for nearly four months does not indicate a higher ratio. This must, however, depend on the relation of its mass to that of the copper, and upon other circumstances not yet ascertained (such as temperature, the relative saltness of the sea, and perhaps the rapidity of the motion of the ship); circumstances in relation to which I am about to make decisive experiments.

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Many singular facts have occurred in the course of these researches. I shall mention some of them, that I have confirmed by repeated experiments, and which have connections with general science.

"Weak solutions of salt act strongly upon copper; strong ones, as brine, do not affect it; and the reason seems to be, that they contain little or no atmospheric air, the oxygene of which seems necessary to give the electro-positive principle of change to menstrua of this class.

"I had anticipated the result of this experiment, and upon the same principle of some others.

"Alkaline solutions, for instance, impede or prevent the action of sea water on copper; having in themselves the positive electrical energy, which renders the copper negative. Lime water even, in this way, renders null the power of action of copper on sea water.

"The tendency of electrical and chemical action being always to produce an equilibrium in the electrical powers, the agency of all combinations formed of metals and fluids is to occasion decompositions, in such an order that alkaline, metallic, and inflammable matters are determined to the negative part of the combination, and chlorine, iodine, oxygene, and acid matters to the positive part. I have shown in the Bakerian lecture for 1806, that this holds good in the Voltaic battery. The same law applies to these feebler combinations. If copper in contact with cast iron be placed in a vessel half full of sea water, and having its surface par

tially above that of the water, it will become coated with carbonate of lime, carbonate of magnesia, and carbonate of soda; and the carbonate of soda will gradually accumulate till the whole surface in the air is covered with its crystals:and if the iron is in one vessel, and the copper forming an arc with it in another; and a third vessel of sea water in electrical connection by asbestos or cotton is intermediate, the water in this intermediate vessel continually becomes less saline; and undoubtedly, by a continuance of the process, might be rendered fresh."

Further Researches on the Preservation of Metals by Electrochemical Means. By Sir HUMPHRY DAVY, Bart.-[1825.]

THE preservation of the copper sheeting of ships from corrosion, by electro-chemical means, having been satisfactorily established by Sir Humphry Davy, he subsequently directed his attention to the important circumstance, how far the cleanness of the bottom of the vessel, or its freedom from the adhesion of weeds, or animals of the polypus and other kinds, would be influenced by this preservation. He says, "As long as the whole surface of the copper changes or corrodes, no such adhesions can occur; but when this green rust has partially formed, the copper below is protected by it, and there is an unequal action produced, the electrical effect of the oxide, submuriate, and carbonate of copper formed, being to produce a more rapid corrosion of the parts still exposed to sea water; so that the sheets are often found perforated with holes in one part, after being used five or six years, and comparatively sound in other parts.

"There is nothing in the poisonous nature of the metal which prevents these adhesions. It is the solution by which they are prevented - the wear of surface. Weeds and shellfish readily adhere to the poisonous salts of lead which form upon the lead protecting the fore-part of the keel; and to the copper, in any chemical combination in which it is insoluble.

"In general, in ships in the navy, the first effect of the adhesion of weeds is perceived upon the heads of the mixed metal nails, which consist of copper alloyed by a small quantity of tin. The oxides of tin and copper which form upon the head of the nail and in the space round it, defend the metal from the action of sea water; and being negative with respect to it, a stronger corroding effect is produced in its immediate vicinity, so that the copper is often worn into deep and irre gular cavities in these parts.

"When copper is unequally worn, likewise in harbours or seas, where the water is loaded with mud or mechanical deposits, this mud or these deposits rest in the rough parts or depressions in the copper, and in the parts where the dif ferent sheets join, and afford a soil or bed in which sea weeds can fix their roots, and to which zoophytes and shell-fish can adhere.

"As far as my experiments have gone, small quantities of other metals, such as iron, tin, zinc, or arsenic, in alloy in copper, have appeared to promote the formation of an insoluble compound on the surface; and, consequently, there is much reason to believe must be favourable to the adhesion of weeds and insects.

"The very first experiment that I made on harbour-boats at Portsmouth proved that a single mass of iron protected fully and entirely many sheets of copper, whether in waves, tides, or currents, so as to make them negatively electrical, and in such a degree as to occasion the deposition of earthy matter upon them; but observations on the effects of the single contact of iron upon a number of sheets of copper, where the junctions and nails were covered with rust, and that had been in a ship for some years, showed that the action was weakened in the case of imperfect connections by distance, and that the sheets near the protector were more defended than those remote from it. Upon this idea I proposed, that when ships, of which the copper sheathing was old and worn, were to be protected, a greater proportion of iron should be used, and that if possible it should be more distributed. The first experiment of this kind was tried on the Sammarang, of 28 guns, in March, 1824, and which had been coppered three years before in India. Cast iron, equal in surface to about th of that of the copper, was applied in four masses, two near the stern, two on the bows. She made a voyage to Nova Scotia, and returned in January, 1825. A false and entirely unfounded statement respecting this vessel was published in most of the newspapers, that the bottom was covered with weeds and barnacles. I was present at Portsmout! soon after she was brought into dock: there was not the mallest weed or shell-fish upon the whole of the bottom

a few feet round the stern protectors to the lead on her ow. Round the stern protectors there was a slight adhesion of rust of iron, and upon this there were some zoophytes of the capillary kind, of an inch and a half or two inches in lengh, and a number of minute barnacles, both Lepas anatif and Balanus tintinnabulum. For a considerable

space round the protectors, both on the stern and bow, the copper was bright; but the colour became green towards the central parts of the ship; yet even here the rust or verdigrease was a light powder, and only small in quantity, and did not adhere, or come off in scales, and there had been evidently little copper lost in the voyage. That the protectors had not been the cause of the trifling and perfectly insignificant adhesions by any electrical effect, or by occasioning any deposition of earthy matter upon the copper, was evident from this that the lead on the bow, the part of the ship >most exposed to the friction of the water, contained these adhesions in a much more accumulated state than that in which they existed near the stern; and there were none at all on the clean copper round the protectors in the bow; and the slight coating of oxide of iron seems to have been the cause of their appearance.

"I had seen this ship come into dock in the spring of 1824, before she was protected, covered with thick green carbonate and submuriate of copper, and with a number of long weeds, principally fuci, and a quantity of zoophytes, adhering to different parts of the bottom; so that this first experiment was highly satisfactory, though made under very unfavourable circumstances.

"The only two instances of vessels which have been recently coppered, and which have made voyages furnished with protectors, that I have had an opportunity of examining, are the Elizabeth yacht, belonging to the Earl of Darnley, and the Carnebrea Castle, an Indiaman, belonging to Messrs. Wigram, The yacht was protected by about th part of malleable iron, placed in two masses in the stern. She had been occasionally employed in sailing, and had been sometimes in harbour during six months. When I saw her in November, she was perfectly clean, and the copper apparently untouched. Lord Darnley informed me, that there never had been the slightest adhesion of either weed or shell-fish to her copper, but that a few small barnacles had once appeared on the loose oxide of iron in the neighbourhood of the protectors, which, however, were immediately and easily washed or The Carnebrea Castle, a large vessel, of upwards of 650 ton, furnished with four protectors, two on the stern, and tw on the bow, equal together to about of the surface of he copper. She had been protected more than twelve month and had made the voyage to Calcutta and back. She came into the river perfectly bright; and when examined in me dry dock, was found entirely free from any adhesion and

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